Aerosol classifiers are used to produce a monodisperse aerosol, that is, they select a narrow range of particles from a larger distribution of particles. This method is used for many applications including; nano-particle generation, measuring distributions of particles in air, measuring the deposition of particles in filters and other devices, sampling ambient aerosols, and many others. These measurements are often done in research areas as diverse as: nano-technology, pharmaceutical research, health-effects studies, inhalation toxicology, bio-aerosol detection, filter testing, indoor-air quality studies, industrial hygiene, energy and combustion research, automotive emissions measurements, and atmospheric and climate-change research.
Currently, the most commonly used classifier is called the Differential Mobility Analyzer (DMA, Knutson and Whitby 1975). The DMA classifies particles based on their electrical mobility, that is, the motion of a charged particle in an electrostatic field. By controlling the electrostatic field and the flow between two cylinders the particles are classified by their electrical mobility, which is related to the number of electric charges on the particle and the drag experienced by the particle, which is a function of the particle's size and shape. For non-spherical particles an equivalent diameter, called the electrical mobility equivalent diameter is defined for these particles, which have the same electrical mobility of a spherical particle of the same size. To classify particles with this instrument an electric charge must be placed on these particles using charging methods such radioactive-source charge neutralizers or corona discharge. However, with all charging methods not a single charge is placed on each particle but rather a distribution of charges are placed on the population of particles. For example, particles may obtain one, two, three, or more positive charges; one, two, three, or more negative charges or no charge at all. The electrical mobility of the particles is a function of the number of charges on the particle and its drag. Therefore, a smaller particle with one charge will have the same electrical mobility as a larger particle with two charges. Thus, the aerosol sample that is classified by the DMA will not be truly monodisperse in terms of particle size, but rather it will have a mix of sizes corresponding to an integer number of charged particles. Techniques are used to minimize the number of charge states but the DMA can never produce a truly monodisperse aerosol. For some applications (like measuring size distributions) the error introduced by the charge distribution can be corrected using inversion techniques, but it can never be fully eliminated. In other applications and experiments, these extra particle sizes can degrade performance or skew results.
Another technique has been used to classify particles by their mass-to-charge ratio is an instrument called the Aerosol Particle Mass analyzer (APM; Ehara et al. 1996; Ehara 1995) or the Couette Centrifugal Particle Mass Analyzer (Couette CPMA; Rushton and Reavell 2004; Olfert and Collings 2005). With these instruments charged particles are classified between two rotating cylinders with electrostatic and centrifugal forces. A similar charging mechanism is applied to charge the particles. Therefore, particles of the same mass-to-charge ratio will be classified. For example, a particle with one charge will be classified at the same time as a particle with twice the mass and twice the number of charges. Therefore, the APM or Couette CPMA do not produce a truly monodisperse aerosol.
Other aerosol and particle instruments are based on measuring what is called the ‘aerodynamic’ diameter of the particle. The aerodynamic equivalent diameter is defined as the diameter of a spherical particle with a density of water that has the same terminal velocity as the actual particle. Instruments that measure the aerodynamic size of particles include various kinds of impactors (Marple et al., 1991; Keskinen et al., 1992), virtual impactors (Conner, 1966), and aerodynamic lenses (Liu et al., 1995a, 1995b). However, these methods only provide a means of dividing the aerosol sample in half, where particles larger than the cut-off point are classified in one direction (i.e., impacted onto the impaction plate) and particles smaller than the cut-off point continue with the flow. Often, several of these stages are stacked together to provide classification into several large bins. There is currently no instrument that classifies particles by their aerodynamic diameter and produces a monodisperse aerosol.